LAMELLIBRANCHIATA. 145 



Hinge-line straight, length aliout one-seventh less than the greatest length 

 of the valve. 



Beaks at about the anterior fourth, prominent and incurved over the hinge- 

 line in the left valve ; depressed in the right valve and not rising above the 

 cardinal line. Umbonal angle nearly rectangular. 



Anterior end large, limited by a broad, slightly oblique, undefined bj'ssal 

 depression ; extremity acute and nasute. Wing elongate triangular, joining 

 the body at the posterior end : the dividing furrow distinct in the left valve, 

 more conspicuous in the right; the fold is not delined in the left valve, and is 

 strongly marked in the right ; margin nearly vertically truncate ; extremity 

 not perceptibly produced. 



Test thin, marked by fine radii, which are conspicuous upon the body and 

 wing, and obsolete on the anterior end and byssal depression. The surface 

 is also marked by fine concentric strias, which are crowded into fascicles, 

 giving an apparent undulation to the surface from the inubo to the lower 

 extremity, especially iu the right valve. At the post-basal margin tlie 

 stria? are recurved at a, little less than a right angle in the left valve, and 

 more acutely in the right valve, and they are strongly laniellose on the 

 anterior end. 



Interior unknown. 



Tlie specimen descril)ed lias a length of 56 mm., height 30 mm., and 

 hinge-line about 4S mm. 



This species resembles P. lata, and also some forms of P. Sao, but it is mucli 

 more elongate, the valves more depressed, the wing narrower and longer. 

 Formation and locality. In the Upper Chemung group at Warren, Pa. 



Ptychopteria lata, n. sp. 



PLATE LXXXV, FIGS. 23-25. 



Shell large, rhomboidal ; body sub-elliptical, very oldique, making an angle 

 with the hinge of nearly 4(1° ; height equal to about two-thirds of the length ; 

 ante-l)yssal margin o)ili([ue, curving into tlie sliallow simis, from wheiu-e tlie 

 19 



